I definitely think a bigger dick makes a difference.
Length gains make a big difference when you begin to reach the cul-de-sac, girth gains make a difference right away (in my opinion) because as it stretches the opening to the vagina, and indirectly increases clitoral stimulation.
There are many stories here where guys have been banging their wives for years, so they know how they respond. When they start to get bigger, their wives start getting more excited, cumming harder, easier and more frequently. Not to mention that I think psychologically, a larger, hard dick excites a woman visually.
Just for fun, for foreplay, try stretching the opening of the vagina, just by pulling down on the labia minora (inner lips)pull down, just gentle tugs, tug outward, tug in different directions. Get the right angle and she will really like it.
To me, the mechanics of a girthier dick does the same thing. By tugging on the labia minora, it pulls the skin over the clit and stimulates the clitoris.
You can just try putting you cock in all the way (especially if your base is the widest part) and just kind of rock in and out, without actually moving you dick, not sliding. It pulses a stretch to the skin over the clit and can even make her cum…especially if the head of your cock is in the cul de sac at the same time.
This technique is refered to as CAT and there are threads here on it.